Multiply every other element in a list

Question

I have a list, let's say: list = [6,2,6,2,6,2,6] , and I want it to create a new list with every other element multiplied by 2 and every other element multiplied by 1 (stays the same). The result should be: [12,2,12,2,12,2,12] .

def multi():
    res = 0
    for i in lst[0::2]:
        return i * 2 

print(multi)

Maybe something like this, but I don't know how to move on from this. How is my solution wrong?

Answer 1

You can use slice assignment and list comprehension:

l = oldlist[:]
l[::2] = [x*2 for x in l[::2]]

Your solution is wrong because:

1. The function doesn't take any arguments
2. res is declared as a number and not a list
3. Your loop has no way of knowing the index
4. You return on the first loop iteration
5. Not related to the function, but you didn't actually call multi

Here's your code, corrected:

def multi(lst):
    res = list(lst) # Copy the list
    # Iterate through the indexes instead of the elements
    for i in range(len(res)):
        if i % 2 == 0:
            res[i] = res[i]*2 
    return res

print(multi([12,2,12,2,12,2,12]))

Answer 2

You can reconstruct the list with list comprehenstion and enumerate function, like this

>>> [item * 2 if index % 2 == 0 else item for index, item in enumerate(lst)]
[12, 2, 12, 2, 12, 2, 12]

enumerate function gives the current index of them item in the iterable and the current item, in each iteration. We then use the condition

item * 2 if index % 2 == 0 else item

to decide the actual value to be used. Here, if index % 2 == 0 then item * 2 will be used otherwise item will be used as it is.